Electrical testing system and method



. Jan. l2, 1932.

Patented Jan. A1.2,V 1932 UNITED STATES PATENT OFFICE ARTHUR J".CHRISTOPHER, OF YONKERS, NEW YORK, ASSIGNOR TO BELL TELEPHONELABORATORIES, INCORPORATED, F NEW YORK, N. Y., A CORPORATION OF NEW YORKApplication led December 6, 1927.

This invention relates to electrical testing or measuring and isparticularly applicable to the determination of phase shift. l

An object of the invention is to Simplify the accurato determination ofthe phase shift characteristics of electrical circuits and apparatus.

In accordance with the invention a simple and cicient arrangement isprovided for measuring phase shift with but a single set-l ting of thebalancing circuit.

A feature of the invention is that a null reading of the meterdetermines that the circuitl is balanced.

" In the form of the invention described herein circuit connections areemployed for balancing the input and output voltages of the circuit orapparatus under test and prov1- sion is made for compensating for thephase difference between the two voltages and for the determination ofthis phase difference. The balancing circuit consists of two variablenon-inductive resistances connected in series and a variable condenserarranged so that it can be connected in parallel with either resistance.The input of the circuit or apparatus under test is connected inparallel with one of the variable resistances. For measurements of phaseshift between 0o and 90o and between 270 and 360 the output is connectedso that if the device to be measured had zero phase shift the voltageacross the output would be in opposite direction to that across theinput. For measurements of phase shift that across the resistance (orresistance and condenser), not in parallel connection with ELECTRICALTESTING SYSTEM AND METHOD Serial No. 238,056.

the input circuit, as indicated by zero deflection in the meter. Thephase shift is determined from the following equation:

tan-1 phase shift angle,

(where R is the resistance connected in parallel with the condenser).

The invention may be more readily understood by reference to thefollowing detailed description in connection with the drawing in whichFig. l shows an embodiment of this invention, Fig. 2 shows a modi- V-voltage induced across secondary winding 3 will be in an oppositedirection to that in primary winding 2, if the transformer has a zerophase shift. An impedance 4 equal to the impedance which the transformeris to work into is connected in parallel with the secondary winding 3and the outer end of this secondary winding is connected to the grid 5of a three-element vacuum tube 6. An impedance 7 equal to the impedancefrom which the transformer is to work is connected between the inner endof the primary winding 7 and one end of a Variable resistance 8. Theother end of variable resistance 8 is connected to the outer end ofprimary winding 2, the inner end of secondary Winding 3 and one end ofvariable resistance 9. The other end of variable resistance 9 isconnected to theptlament 10 of vacuum tube 6. A condenser 11 is arrangedso that it can be connected by means of a switch 18 in parallel witheither variable resistance 8 or variable resistance 9 or leftdisconnected. This con` denser is used to vary the phase dierencebetween the voltage impressed across resistance 9 and the voltageimpressed across wind- 2 of the transformer. Obviousl means 1n ,otlierthan a condenser for varying t e phase could b e substituted. Acrossvariable resistances 8 and 9, which are connected in series,

isv connected the secondary winding 12 ofl vtransformer-13, the primary14 of. which is connected across the source of variable frequency 15. Anamplifier 16 is connected across-the output ofvacuum tubev 6 and an'` l26 up input transformer which at a particular frequency has a zero phaseshift, the test is made as follows: With the condenser 11 disconnectedand with the source of variable frequency 15 connected tothe circuitvariable resistances 8 and 9 are adjusteduntil the reading on the meter17 is the same as was obtained with the source of variable f requency 15disconnected, and the readings of these two resistance values noted.These resistances are to be adjusted so that the value of resistance 8will be small as compared to that of the transformer under test,preferably not greater than 10 ohms. This is desirable in order to keepthe current through the winding 2 as small as possible with respect tothe current magnitude through the resistance 8. If the current throughthe winding 2 is not maintained small with respect to the currentthrough the resistance 8, the reading of the resistance 8 will have tobe corrected for the current oW through the winding 2 for eachadjustment of the resistance 8 and for the impedance of the winding 2ofthe transformer under test. Of course, the lower the value of theseresistances the larger will be the capacity 11 required for adjustmentwhen the phase shift produced by the transformer is other than zero. Thevoltage across variable resistances 8 and 9 is in the same direction asthat across primary winding '2 of transformer 1 and since thetransformer windings are connected so that the voltage across secondarywinding 3 is in the opposite direction to that across primary winding 2,the voltage across the secondary winding 3 is in the opposite directionto that across variable resistance 9. When adjusted so that the readingon meter 17 is the same with the source of variable frequency 15connected as was obtained with the source of variable frequencydisconnected the voltage across variable resistance 9 is equal andopposite to that across the secondary winding 3 and the l' phase shiftother than zero,'it vwill :beimp'ossible to adjust resistances 8 and 9sothat the reading onmeter 17 will bethe samewith the source of. variablefmquency'l connected as was obtained with the source of variablefrequency disconnected, for in that case the voltage across variableresistance 9 cannot be made equal in bothmagnitude and phase to thatacross seconda winding 3' with condenser 11 disconnecteld.y Condenser 11in that case is connected by means of switch 18 in parallel withvariable resistance 8 or variable resistance 9, depending on whether thephase shift produced by the transformer is positive or negative. If thephase shift is positive and between zero and 90, variable condenser 11is connected in parallel with variable'resistance 8 and the circuit isbalanced b adjusting variable resistances 8 and 9 an condenser 11 untilthe phase shift introducedl by condenser 11 is equal to that produced bythe transformer under test and the voltage across resistance 9 equal andopposite to that across secondary winding 3. If the phase shift isnegative and between 27 0 and 360 the circuit is balanced in' a similarmanner with variable condenser 11 connected in parallel with -variableresist ance 9. v

Having obtained readings for variable resistances 8 and 9 and variablecondenser 11 in parallel with variable resistance 8 in the caseof'positive` phase'shift between 0 and 90 the phase shift is equal totan-1 l, 11 The phase shift for a transformer having a negative shiftbetween 270 and 360 is equal to t y R 1 9 tan Xu.

If the phase shift is between 90 and 27 0 it can be determined byinserting, as shown in Fig. 2,l a double pole double throw switch 32vinthe circuit, and connecting the switch Rs When the transformer has apositive phase shift between 0 and 90, condenser 11 is connected inparallel with the resistance 8 and the resistances 8 and 9 and condenser11 are adjusted until the circuit is balanced as indicated by thereading on meter 17. The voltage across variable resistance 9 4is e9;and

e=e3 (where e, equals the voltage across l winding 3). Also e=ae, n(where e...ll is the voltage across variable reslstance 8 and variablecondenser 11 and ,L 1s the amplification of the transformer). Therefore,

elf-M841 but es u varies as Y 'R8-Xiu Rs".7X11

and eo varies as R9. Therefore,

Equation (1) Where the transformer has a negative I phase shift between270 and 360, variable condenser 11 is connected in parallel with4 Var1esas R9 jXll Therefore RgXn afm R8 and Il RRe-J'Xu) In the case of anoutput transformer the phase shift is determined in the same manner asfor the input transformer.

The transmission efficiency of an output transformer can be determinedas follows For an ideal output transformer the resistance looking intowinding 2 is equal to R7 which in turn is equal to (where T equals theturns ratio of winding 3 to windin 2). Then if e equals the voltageacross -8,

Equation (2) equals the voltage across winding 2 (since the resistancelooking into winding 2 is in series with and is equal to R7). Also 6 EXTFor the ideal output transformer l2R 1 R8 a and the ratio R *2 9-RTX'IOO may be used in calculating the elliciencv of an ordinarytransformer in per cent where its phase shift is zero since the lossesin the transformer will result in making Where the transformer has apositive phhse shift between 0 and 90, the impedance of condenser 11which is placed across resistance 8 to obtain balance must be taken intoconsideration. The transmission eiliciency in this case in per cent isequal to 2R9(R8*.7'X11) RgXuT Where the transformer has a negative phaseshift (between 270 and 360) from similar calculations the transmissionelliciency in per cent is 2R9Xi1 v -`.-X1 R8T(R ]Xn) 00 Equat1on(4) Fig.2 shows a modification of the circuit shown in Fig. 1, the circuit undertest being connected to terminals 29, 30 and 31. A.

X100. Equation (3) double throw double pole switch 32 is shown forreversing the input connections of the circuit under test acrossresistance 8. If an input transformer and impedances 'as shown in Fig. 1are connected in the circuit of Fig. 2, when the switch 32 is inposition 19 the transformer 1 and variable resistance 8 are connccted asshown in Fig. 1. Vhen the switch 32 is in position 20, the transformer 1and variable resistance 8 are connected so that the voltage acrossvariable resistance 8 is in the opposite direction to the voltageJacross winding 2.

With this arrangement the measurement of the phase shift can be made incircuits other than transformers where the voltage across the input sideis not in opposite direction to that across the output side, as is thecase of a transformer connected as in Fig. 1. Where the phase shift ofsuch a circuit is between 0 and 90 or between 270 and 360 themeasurements can be made with switch 32 in position 20 and where thephase shift of a circuit is between 90 and 270 it can be measured withswitch 32 in position 19.

In Fig. 2 a transformer 21 is provided with its primary winding 22connected in the circuit where the grid 5 and filament 10 of vacuum tube6 are connected in Fig. 1. This arrangement increases the sensitivity ofthe circuit by permitting transformer.

v Fig. 3 shows a method ofv connecting a transformer 25 in Fig. 2 whenthe transformerA is to be used in a 'circuit which is balanced on oneside and unbalanced on the other or an input transformer designed tooperate .fromvan unbalanced line into pushv vpull vacuum tubes'. Since'the return lead 23 1s connected to the mid-pointgofoutputim-.

pedance 26 only one-hal the volta e across the output winding 27`isbalance agamst the voltage across 'variable resistance 9- or variableresistance 9 in parallelwith variable condenser 11. Where'the out utvWinding 27 consists of two separate win ings they are of courseconnctedl in series aiding. When the output impedance is connectedr asshown in this figure the numerator of e uations 1 to 4 inclusive must bemultiplie by .two to obtain .the amplification `factor or transmissioneiiciency. v

Fig. 4 shows the method of connecting a transformer 33 in Fig.'2 whichis used in a circuit balanced on both sides. R28 is equal to one-halfthe impedance from which the transformer is to work. The equations to beused in calculating the amplification factor or transmission efficiencyare the same as those used when the transformer is connected as in Fig.3.

Although this invention has been described in connection with a specificembodiment it is not to be understood as limited to this particularembodiment since numerous moditications thereof may be madeby personsskilled in the art without departing from the spirit of applicantsinvention, the scope of which is to be determined by the appendedclaims.

What is claimed is:

1'. In a circuit for testing the characteristics of a transformer, anelectrical network comprising a potentiometer circuit and a con- 5denser, said potentiometer circuit having at least two arms, means forconnecting said condenser in parallel with at least one of said arms,means for connecting said transformer to said network, means to producea voltage across partof said network equal and opposite to the voltageproduced across the secondary winding of said transformer and anindicating device for showing when said voltages are equal, saidresistances having impedance values which are small compared to theimpedance of the transformer under test.

2. In a circuit for measuring the phase shift of transformers, apotentiometer circuit for balancing the output voltage of saidtransformer against the voltage across one arm of said potentiometer anda condenser arranged to compensate for the phase difieren-ce betweensaid input and output voltages, at least one arm of said poteniometerhaving a resistance value which is small as compared the usev of astep-'up to'the impedance characteristics of the transformer under test.v f '"f, v

3. A null reading systemof measuring the phase'shift of an electricalcircuit which com- -prises means -for connecting said circuit andanotwork of variablel characteristic com rising elements calibrated 1nreslstance an -re- 'actance to'an alternating current source said"network havingA an arm-,the impedance of which is small compared to the.impedance of thelcircuit under test and the voltage .across which maybeadjustedl to balance thel voltage `acrossthe output'bf said circuit,andmeans for connecting an indlcatorto saidl arm and Asaid electricalcircuit toindicat'e vwhen said voltages 'are equal. c

4. .In a system for measuring the characteristics of an electricalcircuit, anoscillator of variable frequency, a networkcomprising ltwovariable. resistances connected in series cuit equal and opposite to thevoltage across the resistance which is-not in parallel connection withsaid circuit, means for amplifying the Voltage difference between saidcircuit under test and said resistance not in parallel connection withsaid circuit, and means for detecting the amount of said voltagedifference.

5. In a circuit for measuring the characteristics of an electricalcircuit, said characteristics including the phase shift, a potentiometercircuit having two arms, and a condenser arranged to be connected inparallel with either of said arms to compensate for the phase differencebetween the output voltage ofv said electrical circuit and the voltageacross one of said arms.

6. In a system for lmeasuring the phase shift of electrical apparatus, adevice for producing a current of constant effective magnitude, saiddevice comprising a source'of altercuits having a common branch, meansfor connecting said apparatus to be tested in one of said circuits insuch a manner that the voltage across the outputs of said circuits willbe in opposite directions, and means for detecting the difference in thevoltages across the outputs of said circuits.

7 In a circuit arrangement for measuring the characteristics of anelectrical circuit, said characteristics including the phase shift,three variable impedances, one of which is arranged to be. connected inparallel with either of the other two to com nsate for the phasedifference between the output voltage of said electrical circuit and thevoltage across one of said impedances, at least one of said other twoimpedances being small as compared to 'the impedance of the circuitunder test.

8. In a circuit arrangement for measuring the phase shift oftransformers, a circuit comprising two variable impedances for balancingthe output voltage of said transformer against the voltage across one ofsaid impedances, at least one of said impedances having a value which issmall as compared to the impedance of the circuit in which saidtransformer is to be used, and a reactance arranged to compensate forthe phase difference between said input and output voltages.

9. In a circuit for measuring the phase shift of transformers, a networkcomprising variable resistances for balancing the output voltage of saidtransformer against the voltage across one of said resistances and areactance arranged to compensate for the phase difference between saidinput and output voltages, at least one of said resistances having avalue considerably smaller than the impedance of said transformer.

l0. 'Ihe method of measuring phase shift in an electrical circuit whichycomprises applying a voltage to a phase-shifting network, applying avolta e of shifted phase derived from a portion o? said network to theinput of said electrical circuit and determining the phase shift in saidcircuit from the resistance and reactance values of said networkrequired to produce a voltage across another portion of said networkequal and opposite to the voltage across the output of said electricalcircuit.

In witness whereof, I hereunto subscribe

